<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    * {
      margin: 0;
      padding: 0;
    }
    li {
      list-style: none;
      width: 200px;
      height: 100px;
      background-color: chartreuse;
      float: left;
    }
    .content{
      width: 100%;
      height: 200px;
      background-color: coral;
    }

    /* .clear{
      clear: both;
    } */

    .clear::after{
      content: '';
      clear: both;
      display: block;
    }
  </style>
</head>
<body>
  <ul class="clear">
    <li>1</li>
    <li>2</li>
    <li>3</li>
    <!-- <div class="clear"></div> -->
  </ul>

  <div class="content"></div>
</body>
</html>